An assembly mechanism for an external box includes a casing body, a cover body, and a retaining body. The casing body has an opening formed on one side thereof. The cover body is assembled on the casing body. The retaining body is used for sliding the cover body to approach the opening in order to retain the cover body and the casing body together or for sliding the cover body out from the opening in order to separate the cover body from the casing body. Hence, it is easy for users to assemble or detach the external box of the present invention and the cost of the present invention is less than that of the prior art. Therefore, the present invention can solve the problem of needing to use a number of screws to assemble the external box of the prior art.